This invention is concerned with a fixture for laser scribing of dendritic web silicon cells. More particularly, the invention is concerned with a fixture which is used to scribe a laser line onto a dendritic web silicon cell without damaging the components thereof while assuring uniformity from one cell to another while providing for a minimimum of waste consonant with manufacturing techniques adaptable from one state to another.
Conventionally, a dendritic web solar cell is made on strips of single crystal silicon, and the two edge portions which extend along the longitudinal length form dendrites which are to be removed. The cell is typically a p-type (positive type) silicon base material which has a metallized rear contact and a front n-type diffused layer on the silicon base to form a p-n junction. The front of the cell is a metallized electrical contact grid structure (for collecting electrons), and the cell upon exposure to sun produces oppositely charged electrons (-) and holes (+) with the electrons being collected by the grid structure and taken by a wire from the contact grid structure and the holes (+) through a wire on the metallized rear face to transmit current to an external load circuit. The dendrites are formed at the longitudinal edges of a web in a dendritic web process.
A silicon web is formed by the solidification of a liquid film supported by surface tension between two silicon filaments, known as dendrites, which border the edges of the growing strip containing the p-type silicon base.
While costly finishing steps are not required for solar cells made from the dendritic web process, it is still necessary to remove the dendrites at the edges even though the other costly steps of slicing, lapping and polishing are not required. The electrical grid structure is a very fine and precise copper circuit which is electroplated over a thin evaporated titanium, palladium, and silver layer, and once so formed, an improper or a slightly inaccurate cut will mar the face of the cell and destroy the electrical circuit if the copper is cut thereinto. Care also must be exerted so that the p-n junction is not damaged.
Laser scribing has been found to be the most cost-effective because a diamond scribe can cause damage to the cell. Laser scribing is a shallow cut which does not cut through the cell and which provides a sufficient separation such that the dendrites can be easily and cleanly severed from the solar cell.
The invention is more particularly concerned with one of the intermediate stages in the complex process of processing the dendritic web matrix ribbons into dendritic web solar cells, and then having the dendrites removed so that the solar cell can be used as one of the component cells which is placed into a solar panel. More particularly, the invention is directed to the process and apparatus for laser scribing the back or rear face of the cell so that the dendrites can be removed by means of a process and fixture for large cell dendrite removal such as that disclosed in my application as set forth in Attorney Docket 49,556, or equivalent.
The most efficient technique in the process of removing the dendrites is to scribe the periphery of the cell from the back using laser radiation. After scribing, the cells can be separated from each other and the dendrites can be removed from the cell by fracturing the material along the scribe line to remove the dendrites.